1. Field of the Invention
The present invention provides novel radiolabeled lipophilic salts, particularly radiolabeled lipophilic phosphonium and ammonium salts, which are capable of measuring mitochondrial surface potential (ΔΨm). This invention also provides pharmaceutical compositions comprising such radiolabeled lipophilic salts. Additionally this invention provides imaging methods for identifying tissues or cells having aberrant levels of mitochondrial activity by selectively localizing radiolabeled lipophilic salts of the invention into dysfunctional mitochondria. The invention also provides on-invasive methods for an early and sensitive detection of tumor response to chemotherapy agents. The invention further provides treatment methods comprising administration of a high energy radiolabeled lipophilic salts to a patient, particularly patients suffering from diseases or disorders associated with mitochondrial dysfunction.
2. Background
Measurement of the mitochondrial membrane potential (ΔΨm) provides the single most comprehensive reflection of mitochondrial bio-energetic function primarily because it directly depends on the proper integration of diverse metabolic pathways that converge at the mitochondria. Numerous diseases are associated with mitochondria dysfunction, including cancer, cardiovascular and liver diseases, degenerative and autoimmune disorders as well as aging and new pathologies related to mitochondria are identified each year.
Alterations in ΔΨm is an important characteristic of a vast array of pathologies that either involve suppressed (e.g., cancer) or enhanced apoptosis (e.g., HIV, degenerative disease) as well as >100 diseases directly caused by mitochondrial dysfunction such as DNA mutations and oxidative stress (e.g., various types of myopaties).
There are SPECT imaging probes labeled with a technetium center which are capable of accumulation in the mitochondria and the technetium labeled probes have been used for mitochondria based imaging techniques. There are a number of commercially available imaging probes that detect a given pathology using imaging agents such as [99mTc]MIBI, FDG.
[18F]FDG detects malignant lesion due to enhanced glucose metabolism. Further, as mentioned above, [18F]FDG is not able to differentiate neoplasm from inflammation. [18F]FDG is most effective imaging probe for tumor detection but poorly distinguishes neoplasm from inflammation, posing a frequent diagnostic challenge. In certain organs inflammation (e.g., tuberculosis) is a frequent pathologies among patients with suspected malignant lesion. For example, >10% of pulmonary hot spots indicated by [18F]FDG PET are inflammatory process rather than neoplasm, as proven by surgery. In other words, about 10% of lung patients with [18F]FDG PET indications may undergo unnecessary chest surgery, for a disease (inflammation) that otherwise can be treated in non-surgical and less costly and morbid approaches.
Current approaches for evaluation of efficacy of chemotherapy relies on alterations in tumor growth rate, a costly approach of limited sensitivity which involves months of follow up, repeated visits in clinic, multiple radiographic scans and frequently a number of treatment cycles.
Technetium labeled mitochondria imaging agents are hampered by several limitations. More particularly, labeling a molecule with 99mTc requires a conjugating moiety to complex the technetium ion such that Tc-based imaging agents have a high molecular weight which reduces the permeability of the imaging agent in target areas. Further, technetium imaging agents are imaged with SPECT which has relatively low spatial resolution and sensitivity when compared to comparable PET images.
There are technetium complexes, derivatives of [99mTc]annexin V, for apoptosis imaging by using SPECT. The novelty of the proposed [18F]phosphonium cations (PhCs) is that they detect the apoptotic process via a change in ΔΨm, whereas annexin V derivatives do so due to overexpression of specific membrane proteins.
[99mTc]annexin V detects apoptosis due to externalization of phosphatidylserine on the outer cytoplasm membrane. This event occurs at the end of the apoptosis process when the fragmented cell is transformed into clusters of molecules (apoptotic bodies). Shortly after the externalization of phosphatidylserine (termed “eat me” phospholipids) the apoptotic bodies are phagocytized by neighboring cells. Therefore, detection of overexpression of phosphatidylserine is limited to a narrow time window which may last a few days only. Furthermore, the time of appearance and the duration of this window may vary among different chemotherapy agents and subjects.
The collapse of ΔΨm is the point of no return of the apoptotic process. Therefore, the collapse of ΔΨm affords the earliest time point to detect apoptosis, rather the last event as in the case of annexin V, and the collapse persists independent of time.
Current approached for the evaluation of myocardial perfusion and viability have several limitations, including masking of myocardial activity by high accumulation in the organs adjacent to the heart (Th-201, [99mTc]MIBI) and short half-life of the isotope ([13N]-ammonia and 82Rubidum), thus limited to PET centers with an on-site cyclotron.
It would be desirable to have a family of lipophilic salts which have an affinity for mitochondria, particularly mitochondria undergoing aberrant activity.